Hypoplastic nasal bone

Hypoplasia of the nasal bone refers to a sonographic observation where the fetal nasal bone appears smaller by varying degrees.

There is a spectrum of nasal bone hypoplasia, at one end of which is the relatively easily identified absent nasal bone. The other end of the spectrum is considerably harder to define, particularly when used as a screening tool (see below).

Radiographic assessment

Antenatal ultrasound

The nasal bone is best assessed in the second trimester, and its measurement is a standard component of a routine 2nd trimester ultrasound. It is assessed on a midsagittal view of the fetal face. Ideally three echogenic lines should be seen.

The difficulty in defining nasal bone hypoplasia has historically lead to the development of various criteria, based on measurements such as BPD: nasal bone ratio 7, gestational age-adjusted nasal bone length, or a single cut-off definition (0.25 cm) 8.

Many practices now favour the use of data sets that define normal nasal bone length by gestational age, such as that by Obido et al. published in 2007 1. More recently Mogra et al. published a normative data set based on an Australian multiethnic population, and this has now superceded the earlier data sets in some Australian centres 9.